linux-kernel - Re: [PATCH v5 00/12] vfio, dax: prevent long term filesystem-dax pins and other fixes

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Date:   Fri, 2 Mar 2018 14:21:40 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        KVM list <kvm@...r.kernel.org>,
        Haozhong Zhang <haozhong.zhang@...el.com>,
        Jane Chu <jane.chu@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Gerd Rausch <gerd.rausch@...cle.com>,
        stable <stable@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Theodore Ts'o" <tytso@....edu>, Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 00/12] vfio, dax: prevent long term filesystem-dax pins
 and other fixes

On Fri, Mar 2, 2018 at 2:10 PM, Christoph Hellwig <hch@....de> wrote:
> I really don't like these IS_DEVDAX and IS_FSDAX flags.  We should
> stop pretending DAX is a global per-inode choice and get rid of these
> magic flags entirely.  So please convert the instances inside the
> various file systems to checking the file system mount options instead.
>
> For the core ones we'll need to differentiate:
>
>  - the checks in generic_file_read_iter and __generic_file_write_iter
>    seem to not be needed anymore at all since we stopped abusing the
>    direct I/O code for DAX, so they should probably be removed.
>  - io_is_direct is a weird check and should probably just go away,
>    as there is not point in always setting IOCB_DIRECT for DAX I/O
>  - fadvise should either become a file op, or a flag on the inode that
>    fadvice is supported instead of the nasty noop_backing_dev_info or
>    DAX check.
>  - Ditto for madvise
>  - vma_is_dax should probably be replaced with a VMA flag.
>  - thp_get_unmapped_area I don't really understand why we have a dax
>    check there.
>  - dax_mapping will be much harder to sort out.
>
> But all these DAX flags certainly look like a major hodge podge to me.

They are indeed a hodge-podge. The problem is that the current
IS_DAX() is broken. So I'd like to propose fixing IS_DAX() with
IS_FSDAX() + IS_DEVDAX() for 4.16-rc4 and queue up these wider reworks
you propose for the next merge window.

Acceptable?